1. Field of the Invention
This invention relates to circuitry for supplying a known current to a load connected thereto and more particularly to an integrated PNP current source circuit for sourcing a relatively large current to a load coupled thereto.
2. Description of the Prior Art
There are many applications for current sources which require sourcing a large current to a load with a minimal amount of power available to the source to derive the large current. For instance, one such application is the contemporary monolithic electronic ignition system which provides spark for operating an automotive engine. Typically, as understood, the integrated electronic ignition system provides 50 milliamps of current or more to a discrete power device which in turn drives an ignition coil to charge the same so as to generate spark to operate the engine. In the past many electronic ignition systems have used a series pass NPN transistor for sourcing current to the power device. For example, U.S. Pat. No. 3,871,347 discloses the use of a series pass NPN transistor device for driving the amplifier stage which is connected to the ignition coil.
A problem that arises with the use of monolithic ignition systems utilizing NPN series pass devices is the need to meet the automobile manufacturer's requirement that these systems survive an intermittent battery condition while the ignition coil is driving a capacitive load. Under these conditions, large negative current transients occur across the primary winding of the ignition coil. Due to parasitic diodes which are always present in monolithic transistor structures large currents are caused to be sourced from the NPN series pass device which can damage or destroy this device. Another problem occurs under load dump specification conditions wherein 80 volts or more can be applied to the solid state ignition system which makes it very difficult to maintain the series pass NPN device in a safe operating quiescent area without sacrificing system performance. An additional problem caused by these large negative current transients is substrate injection of minority carriers into the monolithic substrate which is caused by the epitaxial substrate parasitic diode present at the collector of the NPN series pass transistor. Substrate injection can cause the integrated circuit to adversely perform and if used in the aforementioned ignition system potentially fail U.S. emission standards.
The above described problems may be overcome by utilizing lateral PNP transistors for sourcing the current at the output of the integrated ignition circuit. However, PNP current sources have disadvantages associated therewith. In order to protect the integrated circuit from load dump conditions if used in an ignition system an internal resistor of 200 ohms or greater is generally required along with zener protection as is known. However, the ignition system must also operate as specified by the automobile manufacturers with a minimum of five volts power supply available. This means that with only five volts for operation that 20 milliamps or less is available to the entire ignition system. Hence, the PNP current source must be suitable for leveraging a small current level up to the 50 milliamps or greater current required to be sourced at the output of the ignition system. However, utilizing today's processing capabilities many, if not all, contemporary high current PNP current sources are susceptible to beta current amplification variations whereby the magnitude of the desired output current may vary over 50% with beta values that can vary from 3-20 due to process variations in the devices at high emitter current densities.
To overcome beta variations and to make the monolithic PNP high current source independent to base current variations caused thereby some contemporary designers have resorted to a base current elimination circuit as understood. This, however, normally requires the use of an operational amplifier which in turn can cause oscillation of the PNP current source as well as other stability problems because of the high gain of the amplifier. To overcome these stability problems many prior art monolithic PNP current sources must utilize an external capacitor to create a dominate system pole at a low frequency such that the circuit has adequate unity gain phase margin.
Thus, there is a need for a high current PNP monolithic current source for multiplying a small reference current up to a large output current which is substantially independent to beta variations of the individual PNP transistors arising from process variations and which requires no external capacitors while exhibiting adequate unity gain phase margin to prevent stability problems.